Traumatic stress is linked to the development of not only posttraumatic stress disorder (PTSD), but also alcohol and drug problems. Both PTSD and problematic alcohol and drug use are associated with increased risk for mortality and other adverse health outcomes. Annual costs are estimated to be $45 billion for PTSD, $185 billion for alcohol abuse, and $145 billion for drug abuse. Nearly half of individuals who have PTSD or a substance use disorder also meet criteria for the other disorder, and these individuals are at even greater risk for poor health and psychosocial impairments. However, the mechanisms that link traumatic stress and PTSD to problematic alcohol and drug use remain poorly understood because extremely few longitudinal, community- based studies contain measures of both pre- and post-trauma functioning. The proposed study offers a unique opportunity to examine the mechanisms of risk that link trauma exposure and PTSD to problematic alcohol and drug use. Data will be drawn from a longitudinal study of familial alcoholism, which follows participants from early adolescence to adulthood and contains measures of both pre- and post-trauma functioning. Given that risk for both trauma exposure and problem substance use peaks during late adolescence/early adulthood, these data are ideal for examining why risk for trauma exposure is so high during this age period, and why trauma exposure is so frequently associated with problematic alcohol and/or drug use. Aim 1 will examine the prospective influences of pre-trauma adolescent and family risk factors (e.g., adolescent alcohol and drug use, family conflict) on risk for trauma exposure, PTSD, and alcohol and drug problems. Findings may reveal differences between risk factors that are most important for preventing trauma exposure, and those that are most important for preventing post-trauma maladjustment. Aim 2 will disentangle the directions of influence among traumatic stress, PTSD, and problematic alcohol and drug use. Results will clarify the extent to which PTSD and alcohol/drug problems share common developmental antecedents, as well as the extent to which traumatic stress influences alcohol and drug problems independent of PTSD. Aim 3 will test whether the long-term influence of trauma exposure or PTSD on alcohol or drug problems is mediated by increases in internalizing or externalizing symptoms. This will be the first known study to longitudinally examine a deviance proneness (externalizing) pathway as an alternative to the self-medication (internalizing) pathway to explain the link between PTSD and problematic alcohol and drug use.